Dr Helen Lang Dept of Geology Geography West Virginia University FALL 2014 GEOLOGY 284 MINERALOGY Mineral Properties in Hand Specimen Luster Metallic Sub metallic Non metallic Vitreous Adamantine Resinous Greasy Silky Pearly Dull Adamantine to sparkle and appear brilliant like diamond Diaphaneity ability to transmit light Transparent Translucent Opaque Color Chromophores Elements that give minerals color Mainly transition metals Fe Ti Cr Mn etc Idiochromatic self coloring Major or necessary elements determine color More likely to be useful for identification Allochromatic other coloring Minor or trace elements determine color Ruby red is from trace Cr in corundum Al2O3 Sapphire blue is from trace Fe and Ti in Al2O3 Staurolite is idiochromatic Rhodochrosite is idiochromatic Staurolite formula is Fe2Al9Si4O23 OH Fe gives staurolite its brown color Rhodochrosite formula is MnCO3 Mn gives rhodochrosite its cherry red color Aquamarine is allochromatic Aquamarine is Beryl formula Be3Al2Si6O18 Trace Fe gives aquamarine its color Structural defects may give quartz color Radiation damage causes purple amethyst or smoky color in quartz Streak is a useful diagnostic property Color of fine powder on streak plate is More reliable than hand specimen color Especially useful for dark metallic minerals Play of Colors Opalescence white light is separated into many colors opal and moonstone Opal Chatoyancy asterism scattering effects Star Sapphire Crystal Form or Shape Special named forms cube Pyrite dodecahedron Garnet octahedron Magnetite Crystal Form or Shape General Shapes prism prismatic platy blocky tabular bladed Crystal Habit in aggregates massive granular radiating acicular dendritic fibrous asbestiform Crystal Habit in aggregates colloform or globular botryoidal bubble like Cleavage Shiny smooth planar breakage Between weakly bonded planes in mineral structure Sometimes hard to tell from growth faces commonly have imperfections not as smooth By quality perfect good fair By shape number and angle between cubic rhombohedral octahedral prismatic Cleavage Examples Fluorite shape called octahedron perfect octahedral cleavage 4 directions of cleavage 8 sides 72 Note difference from Fluorite growth faces cubes More Cleavage Examples Biotite and Muscovite have one perfect basal cleavage and peel off in flexible sheets Hornblende Cleavages in thin section note 56o and 124o angles 60 and 120 Minerals without cleavage have irregular fractures Dr Richard Busch Quartz growth forms Conchoidal fracture in Quartz Hardness relative Mohs Hardness Scale talc gypsum H 1 H 2 knife glass H 5 5 feldspar file streak plate H 6 5 fingernail H 2 5 calcite penny fluorite apatite H 3 H 3 5 H 4 H 5 H 6 quartz topaz corundum diamond H 7 H 8 H 9 H 10 Density or Specific Gravity Density is in grams cubic centimeter g cc Specific Gravity G is mass mineral mass water at 1atm and oC 1 g cc therefore and G values are similar G is unitless Measured by weight in water vs weight in air Density or Specific Gravity Relative density or heft is useful for identifying some minerals most minerals range from G 2 to G 8 average G non metallic minerals 2 7 quartz average G metallic minerals 5 pyrite Barite BaSO4 with G 4 5 feels heavier than most other light colored translucent minerals of the same size Galena PbS with G 7 6 feels heavier than most other metallic minerals of the same size Other Useful Properties A few minerals are magnetic attract a magnet Magnetite Fe3O4 and Pyrrhotite Fe1 xS Some carbonate minerals effervesce fizz in dilute 5 HCl Calcite Rhodochrosite Aragonite others especially Dolomite do not Fluorescence phosphorescence thermoluminescence or radioactivity are diagnostic for a few minerals Twinning When two or more crystals of the same mineral share common atoms typically along planes Twins are characteristic of certain minerals Twinned crystals must be symmetrically related Simple twins only 2 members or parts Complex twins more than 2 members or parts Contact twins share only one plane of atoms Penetration twins members share a volume Polysynthetic twins complex twins with parallel planes of shared atoms Cyclic twins complex twins with non parallel planes of shared atoms Examples of Twinning note re entrant angles which are characteristic of twins Contact Twins Calcite butterfly twin Gypsum swallow tail twin Penetration Twins fluorite staurolite quartz twin spinel twin More twins calcite twin POLYSYNTHETIC PLAGIOCLASE TWINS Best seen thru microscope between perpendicular polaroid filters